This invention relates to a control method of controlling the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine for automotive vehicles, and more particularly to a method of this kind which facilitates smooth transition of engine operating condition between a feedback control region and a mixture-leaning region.
An air-fuel ratio control method for an internal combustion engine is generally known wherein the fuel quantity supplied to the engine is controlled in a feedback manner responsive to an air-fuel ratio correction value set based on an output signal from an exhaust gas sensor which detects the concentration of an exhaust gas ingredient, in order that the air-fuel ratio of the mixture supplied to the engine becomes equal to a desired ratio (e.g. the stoichiometic mixture ratio).
It is also known e.g. from Japanese Provisional Patent Publication (Kokai) No. 59-539 to provide mixture-leaning regions which are defined by engine operation parameters (e.g. vehicle speed, engine coolant temperature, intake pipe absolute pressure, engine rotational speed), and control the air-fuel ratio of the mixture supplied to the engine to a value larger or leaner than the stoichiometric mixture ratio (14.7) while interrupting the feedback control when the engine is operating in any of the mixture-leaning regions, to thereby reduce the fuel consumption.
However, in conventional methods, when the engine operating condition changes from a feedback control region where the air-fuel ratio of the mixture is controlled to the stoichiometric ratio (14.7), to a mixture-leaning region where the air-fuel ratio is controlled to a value leaner than the stoichiometric ratio, the air-fuel ratio is suddenly changed to a value leaner than the stoichiometric ratio so that the engine torque undergoes a sudden change to thereby cause degradation of the driveability. Also, when the engine operating condition changes from a mixture-leaning region to the feedback control region, the same problem occurs due to a sudden change in the air-fuel ratio from the value leaner than the stoichiometric ratio to the stoichimetric ratio.